The present invention relates to an optical multiplexer-demultiplexer, more particularly to an optical multiplexer-demultiplexer incorporating a diffraction grating.
Optical multiplexer-demultiplexers are used in lightwave communication systems to combine optical signals of different wavelengths into a single optical signal, and to separate a wavelength-multiplexed optical signal into its component signals. Conventional optical multiplexer-demultiplexers are of the multiple interference type, employing the phenomenon of constructive and destructive interference among light waves having equal wavelength and various phase relationships. An important parameter of these devices is their wavelength resolution, which depends on their ability to change the propagation direction of an optical signal according to its wavelength.
One example of the prior art is the arrayed waveguide grating (AWG) described by, for example, K. Yukimatsu on pages 137-138 of Hikari Suicchingu to Hikari Intakonekushon (Optical Switching and Optical Interconnections). This device, referred to below as the first conventional device, comprises a closely-spaced array of parallel waveguides of different lengths, passing through two slab waveguides located near the input and output ends of the array. Optical signals interfere in the slab waveguides. The wavelength resolution of this device is expressed by the following equation (1)
dxcex8=xe2x88x92(xcex94l/s)xc3x97(dxcex/xcex)xe2x80x83xe2x80x83(1)
in which xcex94l is the difference in length between adjacent waveguides in the array, s is the spacing between adjacent waveguides at the points at which they are connected to the slab waveguides, and dxcex8 is the angular difference in propagation direction within the slab waveguide of an optical signal of wavelength xcex and an optical signal of wavelength xcex+dxcex. More precisely, xcex8 is the angle between a line joining the entry and exit points of an optical signal and a line normal to the two ends of the slab waveguide, and dxcex8 is the change in this angle caused by a change of dxcex in wavelength.
A desirable goal is a change in propagation direction of at least two-tenths of a radian (dxcex8xe2x89xa70.2 rad) when the wavelength varies by about one percent (dxcex/xcex≈0.01). Structural constraints, however, set a lower limit on the array spacing s and an upper limit on the length difference xcex94l; typically, the array spacing is about fifteen micrometers (s ≈15 xcexcm) and the length difference is at most about forty or fifty micrometers (xcex94l≈45 xcexcm). Accordingly, the change in propagation direction is only about three hundredths of a radian (dxcex8≈0.03 rad) instead of the desired two-tenths of a radian or more (when dxcex/xcex≈0.01).
Another example of the prior art is the device described by H. Nishihara et al. in Hikari Shuseki Kairo (Optical Integrated Circuits). In this device, referred to below as the second conventional device, a diffraction grating is formed ;across one end facet of a waveguide, and an array of optical fibers is connected to the opposite end facet. Optical signals are reflected at different angles by the diffraction grating, according to their wavelengths. The wavelength resolution is given by the following equation (2), in which xcex9 is the grating pitch, xcex8 is the angle of propagation of an optical signal with respect to a line normal to the grating surface, and the other symbols have the meanings given above.
dxcex8=xc2x11/(xcex9/xcexxc3x97cos xcex8)xc3x97(dxcex/xcex)xe2x80x83xe2x80x83(2)
The wavelength resolution of this device can be improved by reducing the grating pitch xcex9, but there are limits beyond which that is not feasible. Typically, the grating pitch xcex9 is about one-third the wavelength xcex, making the maximum change in propagation direction that can be achieved approximately the same as in the first conventional device (dxcex8≈0.03 rad when dxcex/xcex≈0.01).
Still another example of the prior art is a waveguide within which photonic crystals, such as silica particles, are disposed in a regular array. This device, referred to below as the third conventional device, can generate relatively large changes in propagation direction (e.g., dxcex8≈1 rad when dxcex/xcex≈0.01), but it is difficult to manufacture, because the photonic crystals must be very small and the spacing between them must be equal to or less than the wavelength of the optical signal.
Yet another example of the prior art is the device described by J. M. Verdiell et al. in Electronics Letters, Vol. 29, No. 11, pp. 992-993. This device, referred to below as the fourth conventional device, employs a slightly tilted Bragg grating, disposed at one end of a slab waveguide, to reflect light of a particular wavelength to a detector located at the opposite end of the waveguide, thereby selecting one channel from a wavelength-multiplexed optical signal. For operation as a multiplexer or demultiplexer, a separate Bragg grating is employed for each wavelength, the gratings differing in their grating pitch and tilt angle.
The fourth conventional device is also difficult to manufacture, because a separate grating is required for each wavelength.
An object of the present invention is to increase the wavelength resolution of an optical multiplexer-demultiplexer.
A more specific object of the invention is to provide an optical multiplexer-demultiplexer with a structure that is easy to manufacture, in which a one-percent change in signal wavelength produces a change of at least two tenths of a radian in signal propagation direction.
The invented optical multiplexer-demultiplexer has a diffraction grating in which optical signals are reflected by Bragg reflection. The diffraction grating comprises a plurality of mutually intersecting sub-gratings, each having a plurality of perturbing elements, the perturbing elements of each sub-grating intersecting the perturbing elements of the other sub-gratings. Each sub-grating has a grating vector. The diffraction grating as a whole has a combined high-order grating vector equal to a sum of integer multiples of the grating vectors of the sub-gratings. The difference between the propagation vector of an incident optical signal and the propagation vector of the reflected optical signal is equal to the combined high-order grating vector.